Serson's top, which is represented in Fig. 15. con sists of three parts, the top itself, CD revolving on the pivot p at the end of the vertical axis I' p; the agate cup MN in which it revolves, and the appara tus AB by which it is put in motion. The top CD is a hollow cylinder about 21 inches in diameter, and 2 inches high, the upper surface is a speculum, and the lower edge has a small rim inside to make it hea vier. The axis P p is square at top, and the pivot p a cone of about 60', and made of very hard steel. The cup of agate is about an inch in radius. When it is in use the whole is placed in a mahogany box, and a cover of glass is put on to keep off the wind.
In order to put it in motion, the brass frame AB rests with steady pins upon the edge of the mahogany box, so that the spindle S s is immediately above P, the lower end a of the spindle is a hollow square, which receives the square end P of the axis. The spindle S s has always a tendency to rise up by the action of the spring L through which it passes, the four prongs of the spring resting on the top of the frame. A lever E is brought over the knob S, at the top of the spin dle, and keeps the spindle down so as to bring its lower end upon P. A ribband IIG passes through a hole in the lever G, shown separately; and the end G of the ribband being put into the hole K in the spindle S s, the ribband is coiled round the spindle. It is then pulled by the end H, so as to put the spindle and the top in rapid motion, and when G quits K it pulls back the lever GF, and consequently draws the lever E from the top S of the spindle, so as to permit the spindle to rise by the action of the spring L, and quit the axis P of the top, which it leaves in rapid rotation.
This top, as made by more than one of the best art ists, was tried at sea above 60 years ago by some of the first naval officers, but the hopes of success which were entertained were disappointed.
Mr. Weir, many years afterwards, revived this sub ject, and his instrument, made by the order, and at the expense of the board of Longitude, was tried in a king's ship by himself and an astronomer appointed by the admiralty. It was found that when the ship had any motion, the top could not he depended on, to the amount of several degrees, although it performed on shore to a smaller number of minutes. Mr. Weir's machine required a man to keep it in motion while observations were made with it. The reflecting sur
face was fully 12 inches in diameter. The glass rested its weight on a blunt point supported from a chest below. The train of wheels that gave motion to the glass were connected to the latter by means of leather thongs; and the motion of the ship stretching one of the thongs, and relaxing the opposite one, drew the glass from its due position through the angle above mentioned.
About the beginning of 1818, Mr. Troughton began his experiments on the nautical top. His first efforts were very flattering; for by means of an easy adjust ment, he brought the planes of reflexion and rotation parallel to each other, which it requires good work manship to effect. The form which Mr. Troughton gave to the top was that of a hollow cylinder of brass, open at the bottom, and terminated above by a circle of dark glass. The inner diameter of the cylinder was inches, the outer diameter inches, its height 11 inch, and the diameter of the reflecting glass 42 inches. Mr. Troughton afterwards surround ed the cylinder with a solid brass ring, fastened to it by four projecting arms. The upper surface of the ring was on a level with the circle of black glass, which formed the surface of the top; and the inner curved surface of the ring was concentric with the outer curved surface of the top. In this form the top was sent out to the Arctic Regions with Captain Ross, but it did not give such satisfactory results as were expected. Mr. Troughton has since improved it, by giving it the form of an inverted frustum of a cone. The base or lower surface of the frustum is about 6 inches in diameter, the upper surface about 4 inches, and its height about V, inches. The thickness of the metal which forms the cone is 1-8th of an inch. The reflecting plane which occupies the whole upper sur face of the conical frustum, rests in a steel cup half an inch wide, and on a steel point which descends about half an inch below the upper surface of the frus tum. The top is put ill motion by an apparatus ana logous to that used by Serson; but in place of a rib baud, a series of wheels is used, the first and largest of which is put in motion by a winch or handle placed on its circumference. The velocity of the circumfe rence of the base has been calculated at about 30 miles an hour.